Add /migrate slash command

[rka-oai]

Summary

• add a /migrate slash command that injects a canned migration-planning prompt
• add the prompt text and hook the new command into the dispatcher
• document the new slash command in the public docs

Testing

• cargo test -p codex-tui

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https://chatgpt.com/codex/tasks/task_i_690c29d940f88333bc171c36d26436c8

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